1. Field
The present invention relates to image forming apparatuses, and particularly to an electrophotographic image forming apparatus comprising an exposure device which includes an exposure head having a plurality of light-emitting points arranged in a main scanning direction.
2. Description of Related Art
In an electrophotographic image forming apparatus, a photoconductor is exposed to a light beam to form an electrostatic latent image on the photoconductor. In recent years, an exposure device including an exposure head having a plurality of light-emitting points, as implemented by light-emitting diodes (LEDs) or the like, arranged in a main scanning direction (i.e., the direction perpendicular to the direction of transport of a sheet on which an image is to be formed) has been provided for use in this exposure process.
Such an exposure head is typically configured to include a plurality of light-emitting chips arranged in the main scanning direction on a circuit board, and each light-emitting chip may be an LED array chip fabricated through a semiconductor process in which a plurality of LEDs as light-emitting elements are arranged precisely in a single row and packaged in a single semiconductor chip. To be more specific, the LED array chips are arranged in the main scanning direction on the circuit board in such a manner that adjacent LED array chips are in positions shifted from each other in a sub scanning direction that is perpendicular to the main scanning direction so as to prevent a gap in the main scanning direction from being left between a light-emitting point at an end of one chip and a light-emitting point at an opposite end (closer to the one chip) of another chip adjacent to the one chip. Since a single exposure head includes a plurality of chips so arranged that adjacent chips are in positions shifted from each other in the sub scanning direction, and thus the light-emitting points of the LEDs are not aligned in the main scanning direction, a correction may be made to the timing of emission at each of the light-emitting points so that illuminated spots (points of exposure) on the photoconductor are aligned in a straight line. To achieve this correction, usually, amounts of shift of the positions of light-emitting points in the sub scanning direction from a reference line extending in the main scanning direction (such reference line may be predetermined in the exposure head or in the image forming apparatus) are measured, and the data on amounts of correction corresponding to the measured amounts of shift are retrievably stored in the exposure device or in the other portion of the image forming apparatus.
In order to store, as a digital value, the amount of shift in the sub scanning direction of the position of each light-emitting point, the value of the amount of shift may be divided by a quantization unit to obtain a quotient as a digitized (quantized) value of the amount of shift. However, under this scheme, depending upon the actual state of the shift in the positions of the light-emitting points from the reference line, the amount of shift or gap (disparity in position) in the sub scanning direction of adjacent light-emitting points from each other would become unexpectedly great. Accordingly, a correction, if made properly, would yield a desirable result of the amount of shift (gap) in the sub scanning direction of adjacent light-emitting points from each other, that is, not greater than half of the quantization unit; however, if such a correction were made by performing a simple operation of quantization of the amounts of shift, the thus-obtained amounts of correction would possibly yield an undesirable result of the amounts of shift (gap) in the sub scanning direction of a pair (or pairs) of adjacent points of exposure from each other, that is, greater than half of the quantization unit.
In particular, the exposure device with a plurality of light-emitting chips arranged as described above would involve an error (displacement) in relative positions of the chips not only in the sub scanning direction but also in the main scanning direction, when the chips are arranged on the circuit board. Therefore, if a gap in the sub scanning direction is left between a light-emitting point at an end of one light-emitting chip and a light-emitting point at an opposed end of another light-emitting chip adjacent to the one light-emitting chip, a gap in the main scanning direction would be added thereto, and thus a larger gap would possibly be made (which would result in irregularities) in the resulting image formed of points of exposure (illuminated spots).
Under the circumstances, there is a need to provide an image forming apparatus with an improved module for determination of the amount of correction to be made to the timing of emission at each light-emitting point, whereby irregularities, which would appear due to shift or disparity of the positions of adjacent light-emitting points in the sub scanning direction from each other, in the resultant image can be reduced.
The present invention has been made in an attempt to address the aforementioned problem in prior art.